(1) Field of the Invention
The present invention relates to a ceramic foam, which is produced starting from fly ash discharged from a thermal power plant and the like or pulp sludge discharged from a paper mill as main raw materials thereof and is applied to a light aggregate for concrete, and process for producing the same.
(2) Description of the Related Art
In a thermal power plant in which coal is used for boiler fuel, a large quantity of fly ash is discharged as the industrial waste. Recently, such a fly ash has been effectively used as a ceramic aggregate for concrete of building materials. The fly ash is further expected to be put to practical use as reclaimed resources of a large quantity, which is free from care of being exhausted for the time being, as well as the pulp sludge.
For example, Japanese Patent Application Laid-Open No. H10-297977 discloses a technique that powder such as aluminum oxide (alumina) powder is bound to the surface of fly ash followed by sintering so as to produce a hollow ceramic foam.
FIGS. 3A and 3B are a cross sectional view schematically illustrating a ceramic foam 1 before and after sintering, respectively, which is produced by such a conventional process as disclosed in the Japanese Patent Application Laid-Open No. H10-297977. As shown in FIG. 3A, ceramic powder 4 such as Al2O3, MgO and SiO2 powder is coated on the surface layer of fly ash 2 containing impurities 3 such as Fe2O3 followed by granulation so as to form a ceramic powder layer. In a sintering step after the granulation, the granulated fly ash is sintered at 1100-1300xc2x0 C., thereby a ceramic foam 1 having low density and high mechanical strength is produced.
The ceramic foam 1 thus produced according to the conventional process has the following disadvantage.
That is, when the fly ash 2 is coated with the ceramic powder 4 on the surface layer thereof, then granulated and then, sintered, as shown in FIG. 3B, the impurities 3 (such as Fe2O3) contained in the fly ash 2 transfer to the ceramic powder 4 and fuses together with the ceramic powder 4. As a result, a desired ceramic foam, i.e. a hollow particle consisting of two layers in and out, in which a highly pure ceramic shell covers up the surface layer of the fly ash, cannot be produced.
It is therefore an objective of the present invention to solve the above problem and to provide a high quality ceramic foam consisting of two layers in and out, in which a highly pure ceramic shell covers up the surface layer of the fly ash, and a process for producing such a ceramic foam.
In order to attain the above objective, the present invention is to provide a ceramic foam having two layers in and out consisting of: an inner layer of hollow fly ash part produced by a process, in which raw fly ash material containing impurities is coated with alumina slurry on the surface thereof, then granulated and then, sintered; and an outer layer of a ceramic shell part produced by said process covering up the surface of the hollow fly ash part.
According to the constitution described above, a ceramic foam as a high quality hollow particle consisting of two layers in and out, in which a ceramic shell covers up the surface layer of the hollow fly ash, can be provided.
In order to attain the above objective, the present invention is also to provide a process for producing a ceramic foam comprising the steps of: coating the surface of raw fly ash material containing impurities with alumina slurry; granulating the coated fly ash; and sintering the granulated fly ash in a manner that the granulated fly ash is heated in a furnace with raising temperature, then kept at a predetermined temperature for several hours and then, slowly cooled down to room temperature, whereby the impurities are sealed inside the fly ash not to transfer to the outside so that the surface of the fly ash is covered up with a ceramic shell to form the ceramic foam having two layers in and out.
According to the process described above, the impurities contained in the fly ash are prevented from transferring to the alumina slurry, which is coated on the outer surface of the fly ash, and sealed inside the fly ash. As a result, a ceramic foam as a hollow particle consisting of two layers in and out, in which a ceramic shell uniformly covers up the surface of the hollow fly ash, can be provided.
Preferably, in the step of sintering, a rate of the heating is 5xc2x0 C./minute, the predetermined temperature ranges from 1350xc2x0 C. to 1460xc2x0 C., the several hours is three hours, and a rate of the slow cooling is 5xc2x0 C./minute.
According to the process described above, the impurities contained in the fly ash are prevented from transferring to the alumina slurry and from fusing together with the alumina slurry.
Preferably, air is introduced into the furnace with the amount of 100 mL/minute per amount of the granulated fly ash 2.0 g in the step of sintering.
According to the introduction of air with the appropriate amount as described above, a process in which the alumina slurry forms the ceramic shell can be effectively promoted.
Preferably, the alumina slurry has a composition consisting of 65-70 wt % of aluminum oxide, 2-3 wt % of ammonium polycarboxylate as a dispersing agent, 2-3 wt % of acrylic emulsion as a binder, and 25-30 wt % of ion exchanger.
By setting up the composition of the alumina slurry as described above, the ammonium polycarboxylate as a dispersing agent prevents the alumina slurry particles from agglomerateing with each other, the acrylic emulsion as a binder promotes the sintering of the alumina powder, and the ion exchanger acts as a solvent of the alumina slurry, thereby the desired ceramic foam as a hollow particle in accordance with the particle design.